Fundamentals for Interpreting Noninvasive Vascular Testing Part 2

When first encountering the use of duplex ultrasound, and even after acquiring some experience, the phrases “I’m not always sure how to orient the transducer correctly on the patient” and “I’m really not sure how to tell if flow is approaching or receding to identify antegrade or retrograde patterns” may be heard and this can cause some significant distress! So, if you are a newcomer to the field, or if you are refreshing your understanding about image orientation, for both B-mode and Doppler image acquisition and / or interpretation, let’s take a look at the conventions and guiding principles.

Duplex ultrasound transducers (probes) come in a variety of configurations: linear array, curved linear array and phased array sector are utilized most often, with many variations on these designs produced for specific applications. The footprint for each and range of operating (transmit or sending) frequencies are among the factors to consider when selecting a transducer to use for a given duplex examination. This review addresses only transducers that are used externally on the body surface, not endocavity transducers.

Figure 1. Examples of duplex ultrasound transducer types used externally on the body surface

Transducer technology is complex, and a detailed explanation of how each type works is beyond the scope of this article. It is important to keep in mind that lower transmit frequencies penetrate further in terms of depth and higher transmit frequencies produce images with better resolution. Duplex imaging systems use transducers that are constructed to make a range of frequencies available (broadband) in each transducer, allowing more effective use across a range of depths than a single frequency transducer could provide. So, among the options on their machine, the sonographer needs to select a transducer with the highest available frequencies that will provide sufficient depth to reach the target structures and also look at how the transducer will fit on the body in the area that is going to be examined. Additionally, understand that the operating / transmit frequency in any portion of an examination is set separately for B-mode image, spectral Doppler and color Doppler modes and each machine has control settings for adjustment of transmit frequencies in each operational mode . As we are focusing on basic fundamentals, let’s keep it simple. Of the three types of transducers in Figure 1, linear arrays offer the highest range of frequencies, curved linears are designed with lower frequencies, and phased array designed with the lowest frequencies.

Duplex ultrasound systems come with “pre-sets” for various exam types (applications) designed by the equipment manufacturing team to provide a group of settings that are useful for the average patient and for the average perception of what constitutes a good diagnostic image by equipment operators.. This “place to start” will need to be adjusted to optimize images for different patient body types, different disease processes and different operator preferences. Applications specialists for each equipment company can work with equipment operators to establish pre-sets that are more specifically useful or pleasing to each sonographer or for an ultrasound department overall. The sonographer is also able to save settings they use often as additional pre-sets. However, all pre-sets will need to be adjusted for different tissue and flow responses to ultrasound energy during each patient exam as an essential component to producing optimal images. Hence, it is vital to educate ourselves about the settings available on each piece of equipment and, over time, to improve our knowledge continuously on the specific equipment in our clinical sites.

As we move on to the placement of a transducer on a patient, let’s begin by looking at the basic imaging planes before seeing how to position the transducer to produce longitudinal plane views (sagittal, coronal, long axis, vertical) or transverse plane (axial, short axis, horizontal) views.

Figure 2. Standard imaging planes

Each imaging plane (Figure 2) is a 2-dimensional slice through the 3-dimensional body. The vertical planes are in long axis to the body or a part of the body: sagittal divides the right and left halves, coronal divides front and back. The horizontal transverse plane is in short axis to the body and divides it into upper and lower portions. When describing relative position from one slice point to another, we can use the terms superior and inferior, or cranial (head) and caudal (tail or lowest point on the limb or body itself). Additional relative location terminology includes proximal and distal (closer to and further away from some point), and when these terms are applied to the vascular system the reference point is the heart, whether in a longitudinal or transverse approach. Nearer to the heart is proximal compared to a point further from the heart, which is then relatively distal. Medial is used to identify a position that is relatively closer to the sagittal midline of the body and lateral is used when the relative position is away from the midline. Relative position in the coronal plane is referenced using the terms anterior (front) and posterior (back). This review of “location” terminology may appear to be overly basic, however, years of teaching has taught me that assuming we are all speaking the same language can be a significant mistake when it comes to annotating images and documenting findings.

Images may be presented in long axis, transverse or oblique views, with the latter term meaning transducer orientation is not fully longitudinal or transverse to the body or a structure. Skin surface is seen at the point of transducer contact with the body and is displayed at the top of the image with deeper structures displayed progressively toward the bottom of the image. Lack of contact between the transducer surface and the skin, or lack of an adequate ultrasound gel layer between the transducer and the skin, will prevent effective transmission of the sound waves into the tissue and create drop-out in the image display. In Figure 3 a linear transducer was used to obtain a transverse image of the cephalic vein in the mid forearm and the full face of this transducer could not be placed in contact with the skin surface while staying over the vein. The frequency used produced an excellent image but the transducer footprint was wider than the available tissue just medial to the radial bone (radius). The target structure was optimized leaving an image “gap” on the right side of the screen. This should not be mistaken for an imaging artifact, such as shadowing.

Figure 3. Linear transducer with skin contact and position optimized for the vein under investigation on the left and loss of skin contact on the right side of the screen due to the width of the transducer footprint

The display format for linear transducers is basically rectangular, with a flat “top” at the uppermost part of the image screen. Note that a trapezoidal image may be produced electronically with the beam fanned out to produce what are often referred to as “wings” allowing a wider field of view to each side of the image (Figure 4). But the flat top still reflects the face of the linear transducer. Curved linear transducers produce a curve at the top of the image that is congruent with the shape of the transducer face and the bottom of the image is shaped in a wide curve. Using a phased array transducer, the image produced is in a pie slice shaped sector with a narrow point at the top; phased transducers are used in cardiac ultrasound almost exclusively but are also very useful for deep vascular structures or insonating through the rib spaces. Examples of vascular images produced using different transducer types are shown in Figure 5.

Figure 4. Linear transducer with “wings” creating a trapezoidal image. expanding the field of view

Figure 5. Image examples using different transducers and orientations to approach target structures

The sector width field of view may be reduced by the sonographer when using curved linear and phased array transducers to limit the overall image processed for display in order to optimize resolution of the target structures. Narrowing this sector width leads to higher frame rates (number of times per second the image is updated) allowing better temporal resolution in the area of interest. Depth of field should be adjusted when using any type of transducer to optimally display the target structures without having an excess range of depth processed for display beyond the level of the targets, which also improves resolution. Basically, requiring the ultrasound machine to process reflections from regions that are not the focus of your interest will slow down processing overall without yielding any benefit in terms of diagnostic image quality. Color Doppler is a hybrid of Doppler and imaging technologies and the dimensions of the color box should be set to include targets of interest without expansion of the box to cover extraneous regions, also in an effort to maximize resolution.

By convention, as we look at an ultrasound display screen (or a recorded image) the left side of the screen is expected to be oriented to the right side of the patient or to the patient’s head; so conversely, the right side of the screen is oriented to the left side of the patient or to the patient’s foot. Each manufacturer includes an orientation icon at the top of each image. Transducer orientation on the patient needs to correspond to the orientation marker on the screen. For Vascular studies and the vast majority of all ultrasound examination types, this marker on the screen is on the left of the image (Figure 6a). Please note that cardiac ultrasound is the only discipline that does not follow the same image orientation convention, placing the orientation marker on the opposite side (right) of the screen (Figure 6b), requiring a different set of rules for transducer orientation to the screen. In the remainder of this article, description of image / transducer orientation applies to all modalities except echocardiography.

Figure 6A. Orientation marker left side of image, vascular study. Figure 6B. Orientation marker right side of image, cardiac study

The first order of business when we pick up any type of transducer and place it on the body is to determine which end should be oriented to the patient’s head or right side so that the image reads as expected (Figure 7). Just like when reading a map, there are expectations about how north, south, east, and west are displayed.

Figure 7. Directionality in transverse and longitudinal probe placement views when imaging the right Common Femoral Artery (CFA)

Identify the orientation marker on any transducer that you use, either by recognition of an actual mark on the transducer (Figure 8a) or by simply putting gel on the transducer face and touching each end to see which side of the transducer displays your finger imprint on the left side of the screen (Figure 8b). That end should now be oriented to the patient’s head or right side during the duplex examination.

Figure 8A. Orientation marker on a transducer. Figure 8B. Identifying which side of the transducer face should be oriented to the marker on the screen

Once the transducer is oriented correctly on the patient, the relative anatomy seen on B-mode images may be appreciated correctly. Remember that not all views obtained will be in strictly longitudinal or transverse views and may be oblique orientations to the body or structure. The overall goal is to direct the ultrasound beam so that there is optimal reflection from the desired target(s) even if those structures are not laying in a standard plane to the body surface. However, inverting the transducer fully (rotating it 180 degrees) will put structures expected to be on the right side of the screen over on the left side, or structures that are assumed to be toward the head now oriented toward the feet. Figure 9 shows examples of B-mode image orientation in several blood vessels. Note that lateral and medial will change when transverse views are used for structures that are on the right of midline or left of midline.

Figure 9. Screen orientation examples

Doppler assessment of blood flow velocity is done with the transducer in long axis views. Let’s now consider the expected flow pattern in the images presented in figure 10 to appreciate the importance of transducer orientation before moving on to determining flow direction correctly. In figure 10a, the direction of flow in the common carotid artery (CCA) is normally toward the head, and so antegrade flow would be moving from right to left side on the display screen. In figure 10b, the direction of flow in the abdominal aorta is normally directed toward the feet, and so antegrade flow would be moving from left to right side on the display screen.

Figure 10A. Distal common carotid artery (CCA) with bifurcation vessels, internal carotid artery (ICA) and external carotid artery (ECA). Yellow arrow indicates the expected normal direction of flow

Figure 10B. Abdominal aorta. Yellow arrow indicates the expected normal direction of flow

The orientation of the transducer on the skin is of utmost importance when judging direction of flow in the series of tubes that form the peripheral vascular system. Adhering to standard imaging methodology is essential to reading waveform information correctly on the spectral Doppler display of positive (approaching) and negative (receding) flow in relation to the Doppler insonation line of sight . If 0 degree angles to the flow stream were possible in most of the vessels examined in a vascular duplex ultrasound setting, then this discussion of directionality and angle to flow when engaging pulsed Doppler would be a lot shorter, and easier to understand! Flow would be moving vertically, either toward (approaching) or away (receding) from the sound source, which is the Doppler frequency emitted from the transducer. But the horizontal orientation of the majority of peripheral vessels under our transducer does not allow for the “straight down the barrel” approach, with some exceptions that include tortuous bends in peripheral vessels, central upper extremity vessels (see figure 11), perforator veins, abdominal branches from some views and Transcranial Doppler.

Figure 11. PW Doppler using 0?angle with straight, vertical Doppler insonation line of sight. The normal direction of flow in this brachiocephalic vein (innominate vein) would be into the chest, moving away (negative) from the insonation line transmitted from the transducer at skin surface.

Now let’s review an image from the first article in this series to re-establish the terms used with pulsed Doppler (Figure 12), noting that the Doppler line of insonation is angled to the left side of the screen in the vessel. This line may be steered to the left, right or straight down in any image (figure 13), as selected by the sonographer. In addition, the sonographer may also manually heel-toe the transducer on / into the skin and superficial tissues (Figure 14) to make the vessel plane out to be more horizontal on the screen or tilt the vessel more diagonally to one side or the other. The combination of electronic steering of the Doppler insonation line and the mechanical tilt of the probe on the patient is done to achieve acceptable angles (0-60 degrees) for use in generating velocity data. The angle correction cursor is adjusted by the sonographer so that it is lined up parallel with the vessel wall. Angles to flow greater than 60 degrees introduce unacceptable errors when measuring velocities and were not used in the development of criteria charts that guide how we report disease severity. If an angle greater than 60 degrees is entered into the machine and velocity is measured, the error is too big (cosine of the angle is used in the Doppler equation) and therefore invalid for diagnostic purposes.

Figure 12. Identification of electronically produced icons in a duplex image

Figure 13. Different Doppler lines of sight producing 60? angles to vessel walls

Figure 14. Heel-toe manipulation of the transducer

The choices regarding how to direct the Doppler insonation line are in the full control of the sonographer. Likewise, the sonographer controls the angulation and rotation of the transducer on the skin to line up the beam with the intended target structures. The goal is to achieve angles of <60 degrees with the angle correction cursor aligned parallel to the vessel walls at the sample volume site. Combining manual positioning of the transducer through rotation and “heel-toe” angulation of the scan head with electronic steering of the Doppler line of sight, the majority of vessel segments may be interrogated appropriately to generate reasonable estimates of velocity. Figure 15 shows how angulation of the vessel may be achieved by pushing the toe end of the transducer or heel end of the transducer into soft tissue and tilt the target vessel on the screen, positioning it to allow the Doppler insonation line and angle correction cursor to be adjusted at acceptable angles to flow. Whatever angle to flow is entered by the sonographer will be accepted by the machine and used to generate velocity data…..and if that angle is incorrectly entered into the equation the result will be erroneous data.

Figure 15. Achievement of proper Doppler angle of insonation through heel-toe manipulation of the transducer. With credit and thanks to Joanne Sul, RVT for these images

Now we move on to determining direction of flow. Often the question is asked in this way: “is flow coming toward or away from the transducer”? I like the terminology of approaching (coming toward) or receding (going away) from the line of sight based on the way the Doppler insonation line is angled in the displayed image. This may seem like semantics, but confusion seems to reign when it is the transducer that is used to generate the B-mode image and then a Doppler line of sight is employed to gather flow velocity within the image. Yet the flow direction question is often phased in terms of flow direction relative to the transducer rather than the Doppler line of sight. What is meant by the transducer when asking about flow direction is the Doppler line of sight in relation to the target vessel…..so that is what it will be called in this paper.

Just as there are conventions related to positioning the transducer, there are conventions as to how both color Doppler and spectral Doppler are displayed in vascular duplex studies. For consistency and ease of interpretation (the reader may have dozens of images for a single study), it is recommended that normal antegrade systolic arterial flow be displayed above the spectral Doppler baseline and normal antegrade venous flow be displayed below the baseline. It also may facilitate interpretation if normal systolic arterial flow is depicted in red and normal venous flow in blue for color Doppler images. Of crucial importance is to appreciate that the image and color Doppler are frozen at a moment in time when the pulsed wave (PW) Doppler is activated and the waveforms displayed on the spectral grid over time. Even in those cases when “triplex” is used to display B-mode and color Doppler in motion along with the scrolling spectral Doppler, velocity measurements are made when the image is frozen. If flow is all in one direction throughout the cardiac cycle, only colors from one side of the color bar or the other will be displayed. Likewise, if there is no change in flow direction, the spectral Doppler grid will show all waveform components as positive or negative, above or below the baseline. When looking at Doppler display options (figure 16), whichever colors are above the baseline on the color bar are designated for flow approaching the Doppler line of sight and colors below the baseline for flow receding from the Doppler line of sight. For spectral Doppler, flow is either positive (approaching) or negative (receding) with the display option being whether positive or negative flow is shown above or below the zero baseline. On the color Doppler bar, zero baseline is the black region between the red and blue color segments. On the spectral Doppler grid, the zero baseline divides the velocity scale into positive and negative.

Figure 16. Reading color Doppler bar and spectral Doppler scale

Because the Doppler line of sight often needs to change throughout a study in order to properly align the angle correction (AC) cursor parallel to the walls of the targeted vessel segment, both the color bar and the spectral scale may be inverted to keep the conventions working for us in terms of how normal arterial and venous flow are displayed. Equipment manufacturers may include both positive and negative signs on the spectral scale, or just mark the negative side with that mark (-). Some equipment will specify that directionality has been inverted by inserting the word inverted on the spectral Doppler scale, meaning negative is now above the baseline. Look at the examples is Figure 17 in which the direction of flow is determined, and then compared to the normally expected flow direction for a specific vessel. This takes some practice, but is essential to arriving at the correct interpretation of duplex study images.

Figure 17. Determining normal and abnormal flow direction with Doppler. CCA= common carotid artery

Now, let’s look at actual images with flow all in one direction throughout the cardiac cycle (monophasic) in Figure 18 and flow that changes direction in early diastole due to downstream resistance (multiphasic) in Figure 19.

Figure 18. Unidirectional flow (monophasic) throughout the cardiac cycle in a superficial femoral artery (SFA) and profunda femoral artery (PFA). Flow direction is normal, however waveform shape is abnormal

Figure 19. Change in flow direction in early diastole (multiphasic) in a superficial femoral artery due to normal resistance distally at the level of the arterioles Normal direction of flow and waveform shape.

So, when reading flow direction, start by looking at how the spectral Doppler scale is set up in terms of positive flow and negative flow: which direction is displayed above and below the zero baseline. Also note how the color Doppler bar is set up in terms of which color group is designated for flow approaching (coming toward) the Doppler line of sight, and which color group is designated for flow receding (going away from) the Doppler line of sight. Remember that the horizontal axis on the spectral display is time, making spectral Doppler crucial for observing how flow patterns develop over time throughout a number of cardiac cycles. Unless a cine-loop is recorded showing the moving B-mode image and color Doppler changes over a period of time, then still images are captured with image and color Doppler in a frozen view while the spectral trace was developed over time. If a still image is recorded with all three modes displayed (B-mode image, color Doppler and spectral Doppler), the definitive blood flow information is in the spectral Doppler waveform tracing over time and a “flash” moment in time is seen in both the B-mode and color Doppler portions of the captured image.

Once the image set-up is understood with the vessel name and location listed in the annotation, then you may use the line of sight and expected direction of normal flow to determine whether flow is antegrade or retrograde. There are many variations in waveform morphology and the analysis of flow direction is just one of the valuable pieces of information to be appreciated when obtaining and reading Doppler velocity waveforms.

Fundamentals for Interpreting Noninvasive Vascular Testing

Part 2: Basics of Duplex Ultrasound Examinations